*{
    padding: 0px;
    margin: 0px;
}
body{
    height: 1000px;
}
div{
    width: 500px;
    height: 300px;
    background-color: lightblue;
    margin: 50px auto;
    /*对于固定定位来说，父元素与其是没有关系的。只跟着浏览器走*/
    position: relative;
}
img{
    position: fixed;
    bottom: 20px;
    right: 20px;
    width: 100px;
    height: 100px;
}